1. Field of the Invention
The present invention relates to an image sensor which relays image information to an external device.
2. Description of the Related Art
There is a known conventional system in which an image sensor such as a digital camera, a scanner, or the like connected to an external device, such as a computer, relays information collected by the image sensor to the external equipment which operates asynchronous to the image sensor.
In one known structure for such a system, image information is transferred from the image sensor to external equipment while the image sensor captures images and outputs image data obtained by the image pickup based on a control timing generated by the image sensor. In this structure, the image sensor outputs digital image data in units of frames or horizontal lines. Specifically, the image sensor continuously outputs a single frame or a single horizontal line of image data according to a self-generated clock. With this configuration, although the external equipment can instruct the start of reading of image data from the image sensor, once the image sensor has started to output image data, the external equipment cannot readily halt or otherwise control the output of data.
Here, the external equipment, often a device such as a computer, may perform other processing while it receives image data from the image sensor. For example, the device may perform priority processing such as interruption processing or the like. Also, the image sensor is generally configured to be connected to a common bus for the external equipment, and this bus might be used for transmission of data other than that for the image information. Therefore, the structure of an image sensor which unilaterally outputs image data is not preferable for the external equipment. Of special note is the fact that, in this system, the image sensor always outputs image data in frame or line units, even when all that is desired is retrieval of a partial region of data from among the data for a single frame of an image. As a consequence, the external equipment can only send a data reading trigger to the image sensor when its CPU (Central Processing Unit) or bus is free, which occurs only at a certain times, such as after a single frame period or a single horizontal scanning period or more.
In an attempt to solve this problem, there has been proposed a structure in which a frame memory is mounted on the image sensor to temporarily store a single frame of image data obtained by a solid-state image sensor. In this structure, an operation to read an information charge from the solid-state image sensor to store in the frame memory can be performed according to the control timing and control clock generated by the image sensor independent of the state of the external equipment, while the external equipment can access the frame memory of the image sensor according to its own processing stat and the state of usage of the bus, and can read just a desired region of data from a single frame of an image.
However provision of a frame memory is disadvantageous in that the cost of the image sensor increases. There are also problems that it is difficult to reduce the size and the power consumption of the image sensor.